Talc is a mineral used in many applications. In the paper manufacturing industry it is used notably as a filler, but can also be used in paper coatings, or as an additive enabling the quantity of adhesives (pitch or stickies) in the paper manufacturing method to be reduced; it is also used in paints, plastics and filled rubbers, in ceramics, within food and cosmetic formulations and even in agriculture.
Talc can be transported in the form of powder, but it is also suspended in water for reasons relating to ease of transport, unloading and decanting of a liquid product, compared to a powder product. In addition, fine powders also represent a danger for human beings, if inhaled in too large a quantity. The suspension of the talc may be preceded by a step of granulation, by agglomerating individual talc particles in order to form a wet granulate.
However, talc is a highly hydrophobic substance, having surface energy on the order of 68 to 70 J/cm2. Consequently, suspending a substantial quantity of this mineral material in water without however reaching levels of viscosity which would make the suspension unfit for storage or for handling represents a technical challenge for the skilled man in the art. This is reported in “The importance of surface energy in the dispersion behaviour of talc particles in aqueous media” (Powder Technology, 2009, 190 (1-2), pp. 242-246).
The twin problem is therefore faced of increasing what is called the dry extract (or % by dry weight of talc present in the aqueous suspension) of the aqueous talc suspension, whilst giving the latter a rheology compatible with its use (i.e. by limiting the increase of viscosity of the suspension). This latter viscosity may be measured using any technique familiar to the skilled man in the art, such as a Brookfield™ rheometer, a device of the Ford™ cup type, etc.
The choice of the measurement and of the associated device is of little importance, provided they enable the rheological properties of the suspension during storage and/or handling to be apprehended. This independence is recalled in document EP 1 074 293 A1, which covers in very general fashion the dispersal of mineral matters in water, talc being notably mentioned among the substances to be dispersed.
To resolve the problem mentioned above, it is well known to use dispersing agents, which are introduced during the step of suspension of the talc in water, and/or during the step, if applicable, of granulation preceding it.
The skilled man in the art is familiar, on this subject, with document JP 63-175197 which recommends the use of acrylic polymers in a blend with polylakylene glycol esters. A suspension of a dry extract of around 60% is obtained; its rheological properties are studied by means of a sieving test; easy and rapid flow of the suspension is observed for a sieve mesh equal to 150 μm. However, the dose of dry dispersant used is at around 10% by weight relative to the dry weight of talc: the specifications must therefore be put into perspective with regard to this high quantity of dispersant.
Similarly, document JP 02-253836 recommends the use of a (meth)acrylic copolymer in combination with a sulphosuccinic acid. Even if a dry extract of 65% for a Brookfield™ viscosity at 100 revolutions per minute is accomplished of between 300 and 680 mPa·s is obtained, the proportion of dispersant of between 2 and 3% by dry weight relative to the dry weight of talc cannot satisfy the skilled man in the art.
Document FR 2 380 065 A1 is also known, which describes aqueous suspensions of talc having dry extracts of over 65%, and Brookfield™ viscosities at 100 revolutions per minute of the order of 1,000 mPa·s. However, the examples demonstrate that the quantity of dry weight of dispersant must be higher than 1% of the dry weight of talc, and preferentially of the order of 3%. Finally, the dispersant used is a phenol salt, and more specifically a phenol alkyl. And these substances are now prohibited in industry, notably in water-based paints, which constitute one of the potential applications of aqueous suspensions of talc.
Documents U.S. Pat. No. 6,267,811 A1 and WO 2010 055191 A1 are also known, which recommend implementing a CMC (carboxymethyl cellulose)/sodium polyacrylate system. However, the level of performance attained by this pair is modest, in respect of the rheology of the suspension given its dry extract. In addition, this solution has the disadvantage that it uses two components, which increases commensurately the storage and decanting installations (the modified cellulose in this case acts as a wetting agent).
A variant of the previous solution lies in the use of a sodium polyacrylate with a surfactant (GB 2 019 822). Although the characteristics of the resulting aqueous suspension of talc in the sense the present invention are improved, the formation of a very abundant foam is then observed, which is a very prejudicial factor for the final application, since the foam results from the presence of the surfactant in free form in the water. Both the polyacrylate/CMC and polyacrylate/surfactant technical solutions were, incidentally, subjected to comparative testing in the present application.
Currently, it is still the technical solution provided in document EP 0 892 020 A1, which is however more than 10 years old, which provides the best compromise between viscosity/dry extract specifications and low rates of dispersant, without producing a foam. This solution is based on the use of a hydrosoluble copolymer, consisting of a first carboxylic monomer (and notably a (meth)acrylic one) and of a second monomer carrying a grafted surfactant group, where the copolymer satisfies the following formula:R—(OE)m-(OP)n—R′where                m and n designate integers of less than or equal to 100, at least one of which is non-zero,        OE and OP designate respectively ethylene oxide and propylene oxide,        R designates a polymerisable group, and preferentially the methacrylate or methacrylurethane group,        R′ designates a linear or branched alkyl, alkylaryl, arylalkyl, aryl group having at least 22 carbon atoms or a dialkylamine having at least 22 carbon atoms.        
The many examples in this disclosure demonstrate that it is possible to manufacture, with only 1% by dry weight relative to the dry weight of talc of the copolymers described above, aqueous suspensions which have a dry talc extract close to 65% of their weight, and which are at once stable, handleable and compatible with their end uses. The best results are obtained by choosing an aryl group having 30 carbon atoms.